A laser distance measurement apparatus, which may also be called a laser radar, is used for measuring a distance to a measurement object, for example. The laser distance measurement apparatus scans a scanning range including the measurement object by laser light emitted from a laser diode or the like. Based on a photodetector signal outputted from a photodetector that receives the laser light reflected from the measurement object, the laser distance measurement apparatus measures the distance to the measurement object based on the time from the emission to reception of the laser light. The laser distance measurement apparatus is also used for purposes such as obstacle detection for driving support or parking support for a vehicle and peripheral monitoring for detecting a person between doors for automatic platform gates and vehicles at a railroad platform, for example.
The photodetector of the laser distance measurement apparatus receives light with a comparatively wide wavelength band. This causes the laser distance measurement apparatus to also detect other wavelength bands outside the wavelength of the laser light emitted from the laser diode. Especially when the scanning range of the laser light is widened, the photodetector receives the laser light reflected from the measurement object with a wider angle, and accordingly the photodetector receives much more undesirable light such as ambient light. This increases noise due to an influence of the ambient light and the like and deteriorates the measurement accuracy of the laser distance measurement apparatus. To address this problem, a technique has been proposed to suppress the noise due to the influence of the ambient light and the like by providing a band-pass filter, which allows only a wavelength band at and around the wavelength of the laser light to pass, as a preceding stage of the photodetector.
To suppress the noise due to the influence of the ambient light and the like as much as possible, it is desirable that the center wavelength of a passband of the band-pass filter match a center wavelength of the laser light projected from the laser diode. Matching the passband center wavelength of the band-pass filter with the center wavelength of the laser light projected from the laser diode makes it possible to narrow the passband of the band-pass filter.
However, the wavelength of the laser light projected from the laser diode changes to a longer wavelength side at high temperature and to a shorter wavelength side at low temperature. This temperature-dependent variation of the wavelength of the laser light, which is projected from the laser diode in the laser distance measurement apparatus, is approximately 0.3 nm for every 1° C., for example. Hence, when a temperature range of the environment in which the laser diode is used is from −20° C. to 85° C., for example, it is desired that the center wavelength of the passband of the band-pass filter be adjusted to have a wide passband in some measure (31.5 nm, for example) around the center wavelength of the laser light projected from the laser diode.
As other examples of the related art, Japanese Laid-open Patent Publication Nos. 2004-110293, 2013-72771, 2014-95594, 2007-85832, and 6-20079 are known.